Antibodies typically comprise a tetrameric structure having two identical heavy chains that each comprise a heavy chain constant region (CH) fused with a heavy chain variable domain (VH) associated with a light chain constant region (CL) fused with a light chain variable domain (VL). For a typical human IgG, an antibody molecule is approximately about 150 kDa to about 170 kDa in size (e.g., for IgG3, which comprises a longer hinge region), depending on the subclass of IgG (e.g., IgG1, IgG3, IgG4) and (varying) length of the variable region.
In a typical antibody, VH and VL domains associate to form a binding site that binds a target antigen. Characteristics of the antibody with respect to affinity and specificity therefore can depend in large part on characteristics of the VH and VL domains. In typical antibodies in humans and in mice, VH domains couple with either λ or κ VL domains. It is also known, however, that VL domains can be made that specifically bind a target antigen in the absence of a cognate VH domain (e.g., a VH domain that naturally expresses in the context of an antibody and is associated with the particular VL domain), and that VH domains can be isolated that specifically bind a target antigen in the absence of a cognate VL domain. Thus, useful diversity in immunoglobulin-based binding proteins is generally conferred by recombination leading to a particular VH or VL (and somatic hypermutation, to the extent that it occurs), as well as by combination of a cognate VH/VL pair. It would be useful to develop compositions and methods to exploit other sources of diversity.
There is a need in the art for binding proteins based on immunoglobulin structures, including immunoglobulin variable regions such as light chain variable regions, and including binding proteins that exhibit enhanced diversity over traditional antibodies. There is also a need for further methods and animals for making useful binding proteins, including binding proteins that comprise diverse light chain immunoglobulin variable region sequences. Also in need are useful formats for immunoglobulin-based binding proteins that provide an enhanced diversity of binding proteins from which to choose, and enhanced diversity of immunoglobulin variable domains, including compositions and methods for generating somatically mutated and clonally selected immunoglobulin variable domains for use, e.g., in making human therapeutics.